The invention is directed to a high-frequency semiconductor laser module according to the preamble of claim 1.
It is known to use semiconductor laser modules for optical communications technology in the high-frequency range of several GHz or Gbits. Driver circuits for the H-F modulation of the semiconductor laser are usually designed in such a way that the hot H-F connection is negative and the ground connection is positive. A H-F modulating current must be conducted from the driver circuit to the laser diode with as little loss as possible. For this purpose, microstrip lines, coplanar lines or grounded coplanar lines are used. These lines require a dielectric carrier which must have a determined thickness and insulating capability depending on the frequency to be transmitted and depending on the dielectric constant of the carrier. In the case of microstrip lines and grounded coplanar lines, the underside of the dielectric carrier is metallized over a large surface area on the underside of the dielectric carrier at least in the area below the line and is connected to ground potential. Microstrip lines have an individual hot conductor path on the upper side of the carrier. In the case of coplanar lines, the hot conductor is enclosed on both sides at a defined distance by ground lines on the upper side of the carrier. In order to prevent losses, an H-F line must be terminated as exactly as possible with its complex characteristic impedance. Usually, H-F lines with a characteristic impedance of 25 xcexa9, 50 xcexa9 or 75 xcexa9 are used. A laser diode has a complex impedance in the order of 3 xcexa9. For suitable matching of the termination, an ohmic resistance of corresponding magnitude is connected in series. In so doing, it must be taken into account that any parasitic hypothetical resistance brought about for reasons relating to mounting also contributes to the total complex impedance. Because of the low resistance of the laser diode, bonding wires are especially troublesome due to their inductance, while capacitive resistance components contribute only slightly. A further aspect that must be taken into account with respect to the mounting of the laser diode is adequate dissipation of heat loss. For this reason, a laser diode must be mounted on a carrier having the best possible heat conduction.
In consideration of these technical boundary conditions, a number of construction concepts for high-frequency semiconductor laser modules have been developed according to the prior art. When a laser diode is constructed on a substrate with high insulation accompanied by good heat conduction, e.g., aluminum oxide ceramic, almost all of the above-mentioned boundary conditions can be met. FIG. 1 shows a conventional construction of this kind. The laser chip 1 is mounted on a carrier or substrate 2 which is metallized and textured or structured on its surface and which is made of insulating material with good heat conduction, e.g., aluminum oxide ceramic. This carrier 2 carries the electric H-F feeds 41 and 43 at the same time. The hot conductor of these feeds is formed as a microstrip line with an upper metallization 41 and lower metallization 42 and with the carrier 2 as dielectric intermediate layer. The ground line 43 is at the same potential as the back metallization 42. In this respect, it is disadvantageous that a substrate made of ceramic cannot be structured by micromechanical methods with high-precision guide grooves for the optical components required for light coupling such as microlenses or optical waveguides. On the other hand, micromechanical structuring for guide grooves can easily be accomplished by anisotropic etching technique on single-crystal silicon substrates. An example of this is described in DE 3809396 A1.
Apart from its capacity to be micromechanically structured, a carrier made of silicon has the advantage of a very good heat conductivity of 151 W/(m*K). Therefore, heat losses are conducted very favorably from a laser diode that is soldered directly on the silicon carrier. However, the low specific resistance of the silicon substrate of approximately 700 xcexa9cm is disadvantageous. Although there does exist high-impedance silicon with a specific resistance of 6 kxcexa9cm, wafers made from this silicon are roughly one hundred times more expensive than normal low-impedance wafers because of the more complicated production process and are therefore not considered for series production. If the electric lines are to be guided on the silicon substrate, at least one of the feed lines must be constructed so as to be insulated from the substrate in order to prevent short circuiting. Polyimide (PI), for instance, is well-suited as an insulating layer for the electric H-F lines. The PI layer must be thick enough that, in the case of coplanar lines, the H-F field between the lines arranged on the upper side of the PI do not reach the silicon lying below. With grounded coplanar lines or microstrip lines, the silicon substrate is shielded by a metallic base layer at ground potential between the PI layer and the silicon substrate. These lines require, as a dielectric, PI which must have a layer thickness of 10 to 20 xcexcm for a frequency range around 5-10 GHz.
By combining the possibilities known from the prior art for mounting laser diodes on the substrate and for guiding H-F lines, the following two mounting ideas are provided based on silicon as a substrate capable of receiving microstructures:
For example, the laser diode 1 can be arranged on the top of the PI layer 14 (FIG. 2). This would have the advantage that the H-F lines can be guided to the laser diode with low losses. However, due to the fact that the heat conductivity of Pi is inferior to that of silicon by several orders of magnitude, it would not be possible to adequately guide off the heat losses of the laser diode through a PI layer of this thickness.
When the laser diode is placed on a ceramic intermediate carrier 4 which is mounted directly on the silicon substrate 10 (FIG. 3), an adequate dissipation of heat is ensured, but the additional thickness tolerances of the intermediate carrier 4 and its mounting layers negate the advantage of the high positioning accuracy of the microoptical components mounted in the silicon holding structures with respect to the light outlet surface of the laser diode.
It is therefore an object of the present invention to provide a high-frequency semiconductor laser module which avoids the disadvantages of the prior art.
In keeping with these objects, one feature of present invention resides, briefly stated, in a high-frequency semiconductor laser module in which the laser diode is arranged directly on a metallic mounting layer which is located on one side of the silicon substrate, and the two conductors are insulated from one another by the dielectric layer and form the H-F line which is arranged lateral to the laser diode on the same side of the silicone substrate, also via a metallic mounting layer.
The high-frequency semiconductor laser module has the advantage over the prior art that it ensures good heat dissipation from the laser diode and avoids additional high tolerance in mounting the laser, the H-F line, preferably with PI as a dielectric and preferably as microstrip line, coplanar line or grounded coplanar line, is guided close to the laser diode and the laser diode itself is mounted on a metallic laser mounting layer on the silicon substrate.
In many cases, an insulating layer is required between the metallic layer on which the laser diode is mounted and the silicon substrate, as will be explained hereinafter. This insulating layer need not have the thickness that is required for the H-F waveguide because it is located only in the immediate vicinity of the laser diode, so that line losses are only very slight. On the other hand, the insulating layer must have very good heat conduction between the laser mounting layer and the silicon substrate. The insulating layer is advantageously made of silicon nitride and has a thickness between 0.2 xcexcm and 2 xcexcm, preferably 1 xcexcm. Silicon nitride is especially well-suited because it is favorably adapted to the silicon substrate with respect to its thermal expansion (2.8*10xe2x88x926Kxe2x88x921) and also has a sufficient heat conductivity at 25 W/(K*m). In order to improve adhesion, an additional, thinner layer of less than 0.2 xcexcm can be arranged below this silicon nitride layer. This layer is preferably made of silicon dioxide. Because of its low heat conductivity of only 1.4 W/(m*K), the silicon dioxide layer may not be thicker.